Optical coherence tomography (OCT) allows sub-surface imaging along a beam of coherent light providing 1D imaging of the sub-surface structure of the scanned object (a so-called A scan). Some background information on OCT can, e.g., be found in US20140078512.
To apply OCT to 2D and 3D imaging, the beam of an interferometric system is typically moved over the object using an actuated mirror, typically MEMS mirror, such that a sequence of A scans can be assembled to B and C scans which provide 2D and 3D information, respectively, of the scanned object. Such an OCT scanner is a particular type of an optical scanner, however not just acquiring a distance to the scanned object's surface for every beam position, but also an image of the sub-surface structure.
For image reconstruction to be geometrically accurate in the B and C directions, the position of the beam is required to be known to a great accuracy during the acquisition of all A scans. In the prior art, the MEMS mirror position is taken to be a well-defined function of driving voltage, which can be controlled accurately. However, the response of the mirror to the applied driving voltage can change with temperature, ageing of the mirror and any inaccuracy in driving electronics for the mirror. Furthermore, the mirror may display lags due to inertia or hysteresis on its trajectory. Given that MEMS mirrors typically oscillate in the B direction with more than 1 kHz, a forward-only control of their position is thus not optimal.
An optimal position tracking requires the use of some kind of external encoding mechanism. Optical encoders are widely known in the art. Typically, a glass substrate with a well-defined pattern is arranged in a position that is fixed relative to that of the object whose motion is to the tracked, hence following the object's motion, and some stationary photosensitive sensor detects light reflected by the pattern. For example, for a glass scale with equidistant reflecting regions, a position can be established by counting the pulses from the sensor when the glass scale moves relative to the sensor.